Abstract: Well-mixed flow-through experiments are used to study the dissolution kinetics of K-montmorillonite at 25 °C, acidic pH (2-4) and 0.01 M ionic strength. The variations of Si, Al and Mg over time result in high releases of Si and Mg and Al deficit and also in long periods of incongruent dissolution before reaching stoichiometric steady state. 1D RT simulations of the experimental data are performed to quantitatively interpret the evolution of the released cations and to elucidate the stoichiometry of the reaction.